1. Field of the Invention
The present invention relates to wavelength conversion, and more particularly to wavelength conversion using four wave mixing.
2. Description of the Related Art
A wavelength division multiplexed (WDM) signal generally consists of a plurality of optical channels. An optical-electrical-optical wavelength conversion system typically converts a WDM signal from a low frequency band to a high frequency band, or from a high frequency band to a low frequency band using a plurality of photodiodes and laser diodes corresponding to the optical channels of the signal and the desired frequency band for conversion. The use of a large number of photodiodes and laser diodes for such a conversion system requires significant assembly, and consequently increases the size and cost of the system.
In addition, depending on the frequency band of the signal to be converted, laser diodes having a wavelength corresponding to the frequency band to which the signal is to be converted to may not be available. Furthermore, due to the limit in response speed in the optical-electric and electric-optical conversion processes, the transfer speed of the signal is also limited.
Another method of wavelength conversion of a WDM signal comprises four-wave mixing (FWM). FWM combines the WDM signal and pump light in a non-linear medium. Due to the high response speed of an FWM process, it can cope with faster transfer speeds of signal light as compared to an optical-electrical-optical wavelength converter.
Wavelength conversion employing FWM, however, typically generates noise in the frequency bands both above and below the frequency of the pump light in addition to the wavelength converted WDM signal. The noise originates from one of generative FWM (improper) phenomenon wherein at least one of a plurality of optical channels of a WDM signal (before wavelength conversion) is used as pump light for the other channels. This noise generation degrades the quality of a WDM signal following wavelength conversion.
In one embodiment, the invention comprises a method of wavelength converting an optical signal. The method may comprise combining one or more pump lightwaves with said optical signal in a conversion medium, and producing a converted output optical signal spanning a frequency band outside of a frequency band that contains noise signals produced by improper four-wave mixing in said conversion medium.
A variety of specific embodiments are provided and described. In one embodiment, a method comprises four-wave mixing the optical signal channels and a pump lightwave in a conversion medium and selecting a frequency for the pump lightwave such that the frequency difference between any selected optical signal channel frequency and the pump lightwave frequency is approximately equal to or greater than the frequency bandwidth spanned by the plurality of optical signal channels. In another embodiment, a method comprises four-wave mixing, in a conversion medium, the plurality of optical signal channels and first and second pump lightwaves having first and second frequencies respectively. In this embodiment, frequencies for the first and second pump lightwaves may be selected such that (1) the optical signal channel frequencies lie between the first pump lightwave frequency and an average frequency of the two pump lightwave frequencies, and (2) the frequency difference between the first and second pump frequencies is at least about four times the frequency span of the optical signal channels.
In another embodiment, an apparatus for producing wavelength converted light comprises a conversion medium, coupled to receive as inputs a wavelength division multiplexed (WDM) optical signal and a pump lightwave, wherein the WDM optical signal has a bandwidth WBW, and wherein the frequency of said pump lightwave is at least WBW different from the frequencies present in the WDM optical signal.
In another embodiment, a wavelength converter for converting the wavelength of a signal light comprises a first branch having a single pump source and a second branch having a plurality of pump sources.
In another embodiment of the invention, a multifrequency light source comprises N light sources, a multiplexer having the N light sources as an input and a multiplexed output; and a wavelength converter having the multiplexed output as an input and having 2N optical signal outputs.